Some television receivers and monitors have so-called "picture-in-picture" (PIP) provisions for displaying a small picture corresponding to a video signal provided by an auxiliary video signal source within a large or main picture corresponding to a video signal provided by a main signal source. In a television receiver, the main video signal source may be the primary tuner of the television receiver and the auxiliary video signal source may be a second tuner included within the television receiver itself or within an external VCR coupled to the television receiver, or some other composite video signal source such as a VCR.
Television receivers and monitors usually include a beam current limiting (BCL) arrangement which controls contrast and/or the brightness characteristics of the video signals coupled to the picture tube in order to limit the beam current drawn by the picture tube. Beam current limiting is also referred to in the art as automatic beam limiting (ABL). The purpose of a BCL arrangement is to ensure that excessive beam currents, which may cause so-called "white spot blooming" and which may also cause damage to the picture tube, are not produced. A BCL arrangement senses the average current drawn by the picture tube from the high voltage transformer which supplies high operating voltages to the picture tube. Usually, the BCL arrangement generates a control signal for reducing the gain of the video signal processing channel to thereby reduce the peak-to-peak amplitude of the video signals coupled to the picture tube when the level of the beam current exceeds a predetermined threshold. The peak-to-peak amplitude of the video signals is related to the contrast of the reproduced image. The BCL arrangement may also generate a control signal for reducing the DC levels of the video signals coupled to the picture tube and thereby the brightness of the reproduced image.
A copending patent application Ser. No. 695,809, entitled TELEVISION RECEIVER WITH PICTURE IN PICTURE AND NON-LINEAR PROCESSING, filed on May 6, 1991 for R. L. Lineberry concerns a television system with both picture-in-picture and so called non-linear "black stretch" signal processing provisions. A non-linear "black-stretch" signal processing section operates to control the amplitudes of relatively low level video signals between black and a grey level in response to the average video signal level (sometimes referred to as the average picture level or simply as APL). More specifically, the amplitudes of low level video signals are increased for relatively dark scenes (i.e., scenes having low APL) and are relatively decreased for brighter scenes (i.e., scenes having higher APL). In the receiver described in the Lineberry patent application, the non-linear "black-stretch" signal processing operation is disabled during the PIP mode of operation while the small picture is being displayed. As a result, image details of dark scenes of the small picture will not be lost because of the reduction of the amplitudes of low level video signals by the "black-stretch" section in response to a high APL of bright scenes of the big picture.